This invention relates generally to the field of cardiology and, more particularly, to analysis of cardiac electrical activity for identification and therapy of patients at risk for sudden cardiac death.
Analysis of cardiac electrical activity can provide significant insight into the risk state of a patient for sudden cardiac death, and is the major objective in the science of cardiology. Identification of spurious electrical activity within the heart can provide the physician with clues as to the relative cardiac risk presented to the patient. Equipped with such clues, the physician is better able to prescribe therapy for the patient. If a patient is at significant risk, for example, the physician may prescribe pharmacologic therapy, programmed cardiac electrical stimulation, e.g., via an implanted pacemaker or defibrillator, or modifications to the electrical stimulation program in an existing implanted device.
Analysis of T-wave alternans is one mode for identification of sudden cardiac death risk. For purposes of this description, the term xe2x80x9cT-wavexe2x80x9d may refer to a portion of an electrocardiogram that includes the T-wave and the ST segment. T-wave alternans refers to an alternation in the morphology of the T-wave in an AB-AB pattern. In particular, different rates of repolarization of the muscle cells in the ventricles in an alternating pattern have been associated with a variety of clinical conditions including prolonged QT syndrome, acute myocardial ischemia, and elecrolyte disturbance. Nonuniform repolarization can cause electrical instability in the heart. Indeed, T-wave alternans has been recognized as a significant indicator of risk for ventricular arrhythmia and sudden death.
Visual analysis of T-wave alternans using an electrocardiogram ordinarily is impractical due to the minute differences in signal amplitude. At the same time, however, T-wave alternans at even the microvolt level has been identified as an indicator of electrically unstable myocardium. For this reason, computer-based morphology analysis, as well as comparisons in the time and frequency domain, have been used for T-wave analysis. Recently, studies have demonstrated that T-wave alternans, measured upon induction of an elevated heart rate in the patient, is highly predictive of subsequent ventricular tachyarrhythmias and sudden death in patients with a variety of clinical conditions. Such studies typically make use of noninvasive ECG techniques relying on special electrodes that promote noise suppression.
A number of prior art disclosures have been made suggesting techniques for analysis of the T-wave to quantify T-wave alternans, including:
All patents listed in Table 1 above are hereby incorporated by reference herein in their respective entireties. As those of ordinary skill in the art will appreciate readily upon reading the Summary of the Invention, Detailed Description and Claims set forth below, many of the devices and methods disclosed in the patents of Table 1 may be modified advantageously by using the teachings of the present invention.
It is an object of this invention to provide an implantable medical device with an integrated T-wave alternans analyzer for cardiac risk evaluation. It is another object of the invention to provide an implantable medical device that facilitates T-wave alternans analysis without the external equipment such as surface electrodes ordinarily required and, in some cases, without immediate medical supervision. Along those lines, it is a further object to enable T-wave alternans analysis on a more frequent and convenient basis to thereby increase the likelihood of detecting cardiac risks associated with T-wave alternans.
In accordance with the above objectives, a T-wave alternans analyzer may be integrated, for example, with an implantable pacemaker or cardioverter/defibrillator. Integration of the T-wave alternans analyzer with other implantable medical devices such as heart pumps, cardiomyostimulators, ischemia treatment devices, drug delivery devices, and the like also is envisioned. In each case, the T-wave alternans analyzer may be capable of indicating whether the patient is at risk for sudden cardiac death or dangerous ventricular arrhythmias.
In some embodiments, there is provided an implantable medical device having a sensor and a T-wave analyzer. The sensor is implantable within the body of a patient to sense electrical cardiac activity and provide an indication of T-wave alternans within the heart of the patient. The T-wave analyzer is responsive to the sensor, and evaluates cardiac risk based on comparison of T-wave alternans to a predetermined criterion.
In a preferred embodiment, the T-wave analyzer may form part of a microprocessor, a digital signal processor (DSP), or a combination of both. Thus, T-wave sensing can be achieved by conventional analog sense circuitry or the more recently introduced DSP technology. In particular, the device may incorporate analog sense circuitry that processes the analog T-wave signal, or a digital signal processor that performs a similar operation with respect to the digitized T-wave signal, as desired. Digital signal processors have advanced to the degree that cardiac signals may be digitized and analyzed in real time. T-wave signal analysis and comparison to applicable criteria may take place within the DSP or analog circuitry, as well as within processor circuitry resident within the implantable medical device. The T-wave analyzer may be dedicated to T-wave alternans analysis or adapted to perform that function.
The device may include a pacing generator that applies increased rate pacing stimuli to the heart to facilitate sensing of the T-wave alternans by the sensor. In some embodiments, however, it is desirable that the device monitor T-wave alternans on a periodic or triggered basis including times at which the increased rate pacing stimuli has not been applied. For example, the patient may be subjected to exertion or stress in the ordinary course of his daily routine. At such times, it may be desirable to measure T-wave alternans even though the patient has not been affirmatively forced into the measurement mode, e.g., by increased rate pacing. Monitoring of stress and exertion using activity sensors, analysis of heart rate, and the like may provide a trigger for the T-wave alternans analysis.
The device also may incorporate a memory that stores the T-wave alternans indication provided by the sensor, e.g., over a number of heartbeats. The physician may interrogate the memory for more detailed analysis of the T-wave alternans data at a later time. In addition, the device may be equipped to provide an alert to the patient or a physician in the event the processor generates the indication of cardiac risk.
The T-wave analyzer, in some embodiments, may apply known signal processing techniques to quantify and detect T-wave alternans. For example, the T-wave analyzer may analyze T-wave alternans by reference to portions of the electrocardiogram incorporating the T-wave and other components. The T-wave alternans analysis may rely, in some embodiments, on energy, amplitude, magnitude, time and slope differences in the QT interval. The interval can be monitored over a series of two or more alternating heartbeats, for example, to evaluate the cardiac risk.
In addition, the T-wave analyzer may apply a Fourier analysis to the T-wave, and provide the indication of T-wave alternans based on differences in the Fourier analysis over a series of two or more heartbeats. Fourier analysis can be effective in revealing beat-to-beat differences. In some embodiments, the T-wave analyzer can be configured to count the number of times the T-wave alternans satisfies an alternans criterion, and generate an indication of cardiac risk in the event the number exceeds a predetermined threshold.
The results of the T-wave alternans analysis over a period of time can be stored as data in memory for interrogation by a physician, e.g., by telemetry. In response to the alternans data, the physician may prescribe pharmacologic therapy, programmed cardiac electrical stimulation, or modifications to an electrical stimulation program in an existing implanted device.
In some cases, the implantable medical device can be programmed to proactively respond to the alternans data, e.g., by activating an alert for the patient or a physician, modulating a pacing generator, dispensing a drug or other substance from an implanted pump. In this manner, the physician, the implanted device, or both can take action in response to acute and long-term risks indicated by the T-wave alternans analysis.
Various embodiments of the invention are set forth in the accompanying drawings and the description below. Other features and advantages of the invention will become apparent from the description, the drawings, and the claims.